1. Field of the Invention
The present invention relates to an optical material and an optical system using it and, more particularly, to a low refractive index high-dispersion optical material, for example, applicable to optical elements such as lenses, filters, mirrors, refracting optical elements, diffracting optical elements, and so on, and an optical system of optical equipment such as cameras, binoculars, microscopes, and so on, using the optical material.
2. Related Background Art
As one of conventional approaches to correction of chromatic aberration of the optical system composed of only refracting optics, there is a method of combining glass materials of different dispersion characteristics. For example, in the case of objectives of telescopes etc., axial chromatic aberration is corrected by combining a positive lens of a low-dispersion glass material with a negative lens of a high-dispersion glass material. For this reason, there were cases where chromatic aberration was not corrected adequately if the composition or the number of lenses was limited or if available glass materials were limited.
For the purpose of correction of this chromatic aberration, there are suggestions on methods of controlling the refractive index and Abbe's number in order to expand the range of the optical constants of glass materials, thereby obtaining the low-refraction high-dispersion glass materials, for example, in Japanese Patent Application Laid-Open No. 6-32631, Japanese Patent Application Laid-Open No. 61-9262, Japanese Application Laid-Open No. 6-16450, and so on.
On the other hand, other references, e.g. SPIE Vol. 1354 International Lens Design Conference (1990) etc., disclose methods of reducing the chromatic aberration by use of a diffracting optical element provided with a diffraction grating having diffracting action in part of a lens surface or an optical system, in contrast to the methods of reducing the chromatic aberration by the combination of glass materials.
These methods make use of the physical phenomenon in which a refracting surface and a diffracting surface in the optical system demonstrate opposite directions of occurrence of chromatic aberration against rays of a certain reference wavelength.
Further, such diffracting optical element can be provided with an aspherical-lens-like effect by changing periods of the periodic structure of the diffraction grating and is thus greatly effective to reduce aberration.
Comparing them herein as to refraction of rays, a ray appears as one ray even after refracted by a lens surface, whereas a ray appears as a plurality of rays of plural orders after diffracted by a diffractive surface.
When the diffracting optical element is used as a lens system, it is thus necessary to determine the grating structure so as to concentrate light of the used wavelength range into specific orders (which will also be referred to hereinafter as “designed orders”) by adequately enhancing the diffraction efficiency of rays of the designed orders. When light is concentrated in the specific orders, the intensity of rays of diffracted light of the other orders is low. When the intensity is zero, diffracted light thereof appears null. If there exist rays of diffraction orders other than the designed orders, they will be focused at positions different from those of the rays of the designed orders and appear as flare light.
The layouts capable of reducing this decrease of diffraction efficiency are presented in Japanese Patent Application Laid-Open No. 9-127321 and Japanese Patent Application Laid-Open No. 11-44808. The layouts realize high diffraction efficiency in a wide wavelength range by optimizing dispersion properties and thicknesses of gratings of materials different from each other.
Specifically, they disclose the diffracting optical elements wherein a plurality of optical materials (layers) are stacked on a substrate and wherein the diffraction grating is formed in the step shape, kinoform, or binary shape resulting from approximation thereof to the step shape, or the like in at least one of interfaces between the mutually different optical materials.
In each of the diffracting optical elements described in Japanese Patent Application Laid-Open No. 9-127321 and Japanese Patent Application Laid-Open No. 11-44808, the combination of a relatively high-refraction low-dispersion material with a relatively low-refraction high-dispersion material is used in order to obtain the layout having the high diffraction efficiency in the wide wavelength range. Specifically, the materials used are as follows: BMS81 (nd=1.64, νd=60.1: available from Ohara Inc.) and the plastic optical material PC (nd=1.58, νd=30.5: available from Teijin Chemicals Ltd.), in the case of Japanese Patent Application Laid-Open No. 9-127321; and C001(nd=1.5250, νd=50.8: available from Dainippon Ink and Chemicals, Inc.), the plastic optical material PC (nd=1.58, νd=30.5: available from Teijin Chemicals Ltd.), BMS81 (nd=1.64, νd=60.1: available from Ohara Inc.), etc., in the case of Japanese Patent Application Laid-Open No. 11-44808.
For further enhancing the optical performance of the aforementioned diffracting optical elements, we investigated the optical materials commercially available, or the optical materials under research and development and obtained the distribution as illustrated in FIG. 1.
The materials of the stack diffracting optical elements described in the Japanese Patent Applications Laid-Open No. 9-127321 and Laid-Open No. 11-44808 also fall in the distribution of FIG. 1.
As for the materials suggested in the aforementioned Japanese Patent Applications Laid-Open No. 6-32631 and Laid-Open No. 61-9262 and Japanese Patent Publication Laid-Open No. 6-16450, their refractive indexes and refractive-index dispersions are (nd=1.585 to 1.660, νd=40.5 to 32.5), (nd=1.5945 to 1.6925, νd=27.3 to 36.6), and (nd=1.55 to 1.65, νd=27 to 35), respectively, and these materials also fall in the distribution of the existing substances of FIG. 1.
It is seen that 2-ethoxy-ethyl methacrylate (nd=1.483, νd=32) seems only one material with low refractive index and high dispersion and that there have been few such materials developed.